Allogeneic T cell responses drive reactivity to foreign tissues in the setting of solid organ or hematopoietic stem cell transplantation (allo-HSCT). After allo-HSCT, donor alloreactive T cells induce both beneficial graft-versus-tumor activity and harmful graft-versus-host-disease, a life- threatening complication that limits the effectiveness of allo-HSCT. Graft-versus-host-disease is a serious medical problem for which existing therapeutic interventions are often ineffective. In addition, existing strategies to control graft-versus-host disease impair anti-tumor responses, leading to an increased risk of cancer relapse. Discovering novel immunomodulatory approaches to control the harmful effects of allogeneic T cell responses without eliminating their beneficial anti-cancer activity is essential to improve the long-term success and widespread applicability of allo-HSCT. We have identified a new critical role for Notch signaling in alloreactive T cells mediating graft-versus-host disease after allo-HSCT. Inhibition of canonical Notch signaling in donor T cells markedly reduced the severity and mortality of graft-versus-host disease in several mouse models of allo-HSCT. Notch-deprived T cells proliferated normally and showed increased expansion in lympho-hematopoietic organs, demonstrating the absence of global immunosuppression. Notably, Notch-deprived alloreactive T cells acquired efficient cytotoxicity in vivo and retained potent anti-leukemia activity, leading to markedly improved overall survival of the recipients. However, their ability to produce multiple inflammatory cytokines was reduced. Notch inhibition also decreased the accumulation of alloreactive T cells in the intestine, a key GVHD target organ. Thus, Notch signaling represents a promising therapeutic target to control graft-versus-host disease while preserving significant anti- cancer activity in donor T cells after allo-HSCT. We hypothesize that Notch is a new essential regulator of T cell function in allogeneic T cell responses. To explore this hypothesis in detail, we will determine the specific Notch ligands and receptors that mediate Notch activation in T cells after allogeneic HSCT; investigate the cellular and molecular mechanisms underlying the decreased induction of GVHD by Notch-deficient alloreactive T cells; and identify the cytotoxic pathways that mediate the persistent anti-cancer activity of CD4+ and CD8+ T cells upon Notch inhibition. These studies will bring novel insights into the molecular regulation of alloimmunity and might lead to the development of new approaches to limit damaging consequences of T cell reactivity after allogeneic transplantation. .